1. Technical Field
The invention relates generally to blowout preventers and, more particularly, to a ram-type blowout preventer having shear rams for shearing a pipe, casing, or other oilfield tubular.
2. Background Art
During well drilling operations, fluid may flow into the well from subsurface formations adjacent the well. If the formation fluid influx is not properly controlled, the well may blow out. Thus, blowout preventers are usually installed at the wellhead to contain pressure in the wellbore and prevent the well from blowing out while the formation fluid influx is controlled. A ram-type blowout preventer has a bore that may be aligned with the well and a pair of opposed rams that may be actuated to engage each other and close off the bore. The rams may be shear rams which carry blades that can shear a pipe, casing, or other tubular that is suspended in the bore of the preventer. Typically, the pipe is sheared by moving the rams against the pipe to substantially flatten the pipe at the blade contact region. Further movement of the blades against the pipe then shears the flattened portion of the pipe.
In certain instances, such as when the diameter of the blowout preventer bore is much less than half of the circumference of the pipe, the length of the flattened-out portion of the pipe may interfere with further travel and shearing action of the rams. The flattened-out portion of the pipe may also wedge in the preventer bore such that removal of the pipe and control of the well is seriously impaired. Therefore, it is desirable to have a shear ram that will cleanly shear any diameter of pipe that can be run into the bore of the preventer. It is also desirable that the shear ram shears the pipe in a manner that will not impair pipe removal and well control procedures.
U.S. Pat. No. 5,400,857 to Whitby et al. discloses a ram assembly for positioning in a blowout preventer which includes opposing V-shaped blades that are arranged to constrain a tubular in the bore of the preventer prior to shearing the tubular. The V-shaped blades are moved radially inward to engage the tubular at four contact points and deform the tubular to a rectangular-shaped configuration. After deformation of the tubular, further movement of the blades against the tubular applies forces which creates stress fractures in the tubular. The stress fractures propagate to essentially result in brittle shearing of the tubular.